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Benzioni, A., S. Mendlinger, M. Ventura, and S. Huyskens. 1993. Germination, fruit development, yield, and postharvest characteristics of Cucumis metuliferus. p. 553-557. In: J. Janick and J.E. Simon (eds.), New crops. Wiley, New York.

Germination, Fruit Development, Yield, and Postharvest Characteristics of Cucumis metuliferus*

A. Benzioni, S. Mendlinger, M. Ventura, and S. Huyskens

  1. METHODOLOGY
    1. Germination Experiment
    2. Date of Planting Experiment
    3. Fruit Development
    4. Fruit Constituents
    5. Ethylene Treatments
    6. Storage
  2. RESULTS
    1. Germination
    2. Yields
    3. Fruit Development
    4. Ethylene Application
    5. Storage
  3. CONCLUSIONS
  4. REFERENCES
  5. Table 1
  6. Table 2
  7. Table 3
  8. Table 4
  9. Fig. 1
  10. Fig. 2
  11. Fig. 3
Cucumis metuliferus Mey. (the African horned cucumber, kiwano, melano) is endemic to the semi-arid regions of southern and central Africa, where it is eaten as a supplement by the local population (Bruecher 1977; Keith and Renew 1968). The plant is a monoecious, climbing annual with staminate flowers typically appearing several days before pistillate flowers. The ellipsoid fruit is bright yellow-orange in color when mature and shaped like a short stout cucumber with many blunt thorns on its surface. The mesocarp is green and consists of juicy bland-tasting tissue. Cucumis metuliferus is exported as a speciality fruit from New Zealand, Kenya, and Israel to Europe, and its market is expanding. Several problems associated with agrotechniques and fruit quality have emerged in the course of commercialization. Some of the problems encountered include poor germination, too small fruits at some locations, rapid fruit deterioration during cold storage under humid conditions, and failure of fruits to develop the desirable uniform orange color. In this paper, we report on efforts to improve conditions for successful germination, the effect of sowing dates on crop development, as well as fruit ripening in the field and in storage, and the effects of ethylene application to fruits in order to establish optimum harvest time, storage, and shipping conditions.

METHODOLOGY

Germination Experiment

Percentage and rate of germination were examined at 8°, 12°, 20°, 25°, 35°, 40°, and 45°C. Seeds were placed between two layers of wet cotton in petri dishes that were scored daily over a period of 24 days for germination. Germination under saline conditions of 0, 20, 50, and 80 mM NaCl was examined at 30°C .

Date of Planting Experiment

Seeds were sown on sandy loess at the Sha'ar Hanegev Experimental Station (northern Negev, Israel) on Mar. 15, Apr. 15, and June 3, 1988 at a density of 10,000 plants/ha. The field was dripirrigated twice a week at 2 liters/hr, with amounts calculated to replenish 40% of evapotranspiration till first flower stage, and 80% thereafter. Cultural measures (fertigation and disease and pest control schedules) were similar to the local practice for melons. A single harvest when almost all fruits were ripe (after color break, i.e. when skin shifts from dark green to pale yellow), was carried out on June 30 and on Sept. 16 for the first and second sowing dates, respectively. Plants sown in June failed to produce fruit prior to October and thus, were not harvested. At each harvest, a 10-m2 plot was harvested from each bed and all fruits collected and divided according to fruit weight into large (>200 g) and small (<200 g).

Fruit Development

About 65 flowers were tagged and hand pollinated at anthesis. Fruits were picked 33, 37, 45, 51, and 61 days after pollination (DAP) for determination of fruit constituents.

Fruit Constituents

Total soluble solids (TSS), pH, electrical conductivity (EC), and reducing sugars (Sumner 1921) were determined in the fruit jelly. Pigments were extracted from the peel by a 4 acetone:5 hexane mixture.

Ethylene Treatments

Fruits were picked when whitish green, at about the turning point. Ethylene was applied by exposing fruits for 24 h to 160 µl ethylene. Treated and control fruits were then stored at 20°C for two months for further ripening and samples of fruits were removed periodically for analysis.

Storage

Yellow-orange fruits were stored at temperatures of 4°, 8°, 12°, 20° and 24°C. In addition, green fruits at about the turning point were picked and stored at 20°C. Every week, fruits were evaluated and overripe, soft, or damaged specimens were discarded.

RESULTS

Germination

Optimal temperatures for germination were 20° to 35°C where 95 to 100% of the seeds completed germination in three to eight days (Table 1). At 12°C, germination commenced at day 16, reaching a final count of 90% on day 24, and at 8°C was completely inhibited. At very high temperatures (40° and 45°C) percentage germination was greatly reduced, although enough seeds germinated to indicate possible genetic variation for heat tolerance (Table 1). Germination was unaffected by salinity of up to 50 mM NaCl.

Yields

March sown plants covered the beds in about six weeks and began to flower and set fruits in eight weeks. Vines of plants sown on the second date (Apr. 15) covered the beds only after ten weeks, although here too flowering began eight weeks from sowing. Fruit development was slower in the April sowing and although the number of fruits/ha was similar (271,000 versus 245,000 for the March planting), many fruits were smaller (Table 2). The plants sown on 3 June failed to grow well, and by the end of the experiment in October had neither covered the beds nor flowered. The plants of the March sowing yielded over 46 Mg/ha of fruits (Table 2). More than 60% of these fruits were large (>200 g) and would command premium prices in the market. The yield for the second sowing date was 28 Mg/ha, of which only 25% consisted of fruits classified as large; nearly half the fruits were very small and of non-commercial size (Table 2).

Fruit Development.

Maximal fruit weight of 205±;57 g was reached by 33 DAP. The main period of fruit ripening on the plant in terms of changes in fruit constituents and color occurred between 37 and 51 DAP (Fig. 1, 2). During this time, both total soluble solids (TSS) and reducing sugar levels increased in fruits attached to the plant, peaking at about 50 DAP (Fig 1). The ripening period was also characterized by changes in the color of the fruit peel, with absorbance of light at wavelengths of 663 and 431 nm decreasing with time indicating loss of chlorophyll, and with absorbance at 442 and 470 nm increasing, indicating carotenoids production (Fig. 2).

Ethylene Application

Application of ethylene to fruits at the breaker stage resulted in fruits progressing from green to yellow within three days of treatment (Fig. 3). Non-ethylene treated fruits took much longer to change color and at day 60 still appeared slightly green and less mature than ethylene-treated fruits. From the changes observed in the absorption peaks during ripening, it appears that in ethylene-treated fruits the absorbance at 431 nm disappeared three days after treatment while it took more than 30 days to fade in untreated fruits. The decline in the 431 nm peak was followed in all fruits by the development of a peak at 442 nm (Fig. 3). In some of the ethylene-treated fruits a new absorption band appeared at 420 nm, and fruits were more orange than yellow. No absorption at this wavelength was detected in fruits of the control group (data not presented). The TSS levels remained stable during storage and were unaffected by ethylene treatment. Reducing sugars of both treated and non treated fruits rose during storage from 16 to 25 to 30 mg/g fresh weight (Table 3). The rise in reducing sugars during storage was not enhanced by ethylene treatment and occurred within 12 days of commencement of storage. Electrical conductivity (EC) did not change during storage, and a decline in acidity and a rise in pH were observed (Table 3). Fruits from the same experiment allowed to ripen in the field had a higher final content of reducing sugars on a fresh weight basis, 42 mg/g vs 25 to 30 mg/g in storage (Table 3).

Storage

Fruits picked after the initiation of ripening (yellow color detectable) kept well at 20° and 24°C. All fruits kept at 24°C and 70% of those kept at 20°C were still firm and undamaged after three months of storage (Table 4). Cold storage at 4° or 8°C resulted in much shorter shelflife, and at 4°C chilling symptoms in the form of opaque spots on the fruit surface appeared (Table 4).

CONCLUSIONS

The best time for sowing C. metuliferus at our sites would be mid-March to early April. Fruits require about 35 days to mature green and another two weeks to full maturity. Ethylene enhances ripening similarly to its effect on 'Honey Dew' melons and its application may be of agronomic use, enabling growers to harvest at the breaker stage and store fruit for a longer period (McGlasson and Pratt 1964; Pratt et al. 1977). Fruits could be treated with ethylene just before marketing to induce a pleasing, uniform orange color on their arrival at the market.

The fruit has an exceptional long shelf life at temperatures of 20° to 24°C which makes it eminently suitable for development as a new exotic or ornamental crop. However, further research aimed at breeding or selecting for tastier genotypes is essential since the present unsatisfactory taste limits the potential market.

REFERENCES

*This work was supported by the GIARA program file No 864393. We gratefully acknowledge the excellent technical work of S. Avni and the styling of A. Sen and I. Mureinik.
Table 1. Germination percentage of Cucumis metuliferus seeds at eight temperatures and four NaCl concentrationsz.

Variable Days till
maximum
germination		 Final
germination
(%)
Temp. (°C)
8 --- 0
12 24 90
20 8 100
25 3 100
30 3 100
35 5 100
40 20 35
45 2 15
NaCl (mM)
0 3 100
20 3 100
50 12 100
80 24 100
zGermination tests used 10 seeds and two replications (15 seeds at 30°C for NaCl test)

Table 2. Fruit size fruit number and fruit yield of Cucumis metuliferus at different sowing datesz.

Yield (Mg/ha) Fruit number (1000/ha)
Sowing date >200 <200 Total >200 <200 Total
March 15 27.9a 18.5a 46.4a 107a 138b 245a
April 15 7.0b 21.2a 28.2b 31b 239a 271a
zMean separation in columns by Duncan multiple range test, 5% level.

Table 3. Effect of ethylene (160 ppm), applied for 24 h, on fruit constituents during storage. Values are means ±SE for 5 to 7 fruits.

Maturity at harvest Storage (days) Ethylene trt. Fresh wt. (g/fruit) pH EC (dS/m) Acidity (µeq/gfw) TSS (%) Reducing sugars (mg/gfw) Water loss (%)
Mature green 0 225±12 4.06±0.02 4.82±0.30 91±5 4.76±0.19 16.10±0.70 0.0
Mature green 3 + 198± 5 4.15±0.04 4.14±0.17 96±8 4.57±0.21 22.08±2.27 0.5
Mature green 199±12 4.22±0.05 4.11±0.1 0 88±6 4.33±0.19 19.96±1.02 0.8
Mature green 12 + 234±15 4.12±0.04 4.09±0.22 103±5 4.53±0.19 28.42±0.96 1.3
Mature green 202±17 4.21±0.06 3.72±0.12 85±9 4.02±0.38 21.12±2.60 1.4
Mature green 25 + 214±20 4.21±0.05 4.22±0.12 82±8 4.60±0.29 30.04±3.50 3.3
Mature green 219±10 4.18±0.05 4.14±0.1 6 81±8 4.93±0.30 30.18±4.04 2.8
Mature green 61 + 212±14 4.29±0.03 3.79±0.49 74±2 4.40±0.07 26.84±1.06 7.6
Mature green 211± 8 4.89±0.05 4.11±0.32 76±8 4.73±0.49 25.56±1.88 6.6
Yellow orange 0 236±10 4.24±0.02 3.41±0.14 96±3 6.19±0.14 41.62±3.20 0.0

Table 4. Effect of storage temperature and maturity at harvest on shelf life of Cucumis metuliferus. At each temperature 24 fruits were scored once a week; spoiled fruits were discarded.

Spoiled fruits (%) after storage
Maturity at
harvest		 Storage
temp. (°C)		 30d 37d 45d 55d 75d 90d
Ripe 4 17 89 100 100 100 100
Ripe 8 25 58 92 100 100 100
Ripe 12 60 60 100 100 100 100
Ripe 20 0 30 30 30 35 35
Ripe 24 0 0 0 0 0 0
Mature green 20 0 0 0 0 0 0
Mature green 24 0 0 0 0 0 0

Fig. 1. Accumulation of reducing sugars and changes in soluble solids (TSS) in fruits maturing in the field. Fruits were picked at different stages of development. Values are means ±SE of five fruits.
Fig. 2. Pigment profile changes in the fruit peel during fruit development in the field. Values are means ±SE of 5 fruits.

Fig. 3. Effect of 24 h of exposure to ethylene on pigment profiles of fruits during storage at 20°C. Each value is the mean of 5 to 6 fruits. Bar indicates the critical range according to the Tukey test.
Last update September 17, 1997 aw